1. Field of the Invention
The present invention relates to the estimation and monitoring of timing errors from packet data networks. More specifically, the present invention relates to a system and method for estimating timing/phase wander and for regenerating T1/DS1 clocks using information derived from timing pseudowire or packet data flows.
2. Background Art
Those of ordinary skill in the art of the present invention appreciate that there are problems in the current state of technology with time division multiplexing (TDM) timing recovery for a wireless network that uses TDM clocks regenerated from pseudowire packet data flows. Many networks rely on the timing accuracy of the regenerated TDM signals and demand a means of determining if the timing is correct.
Historically, cellular networks, and other communication equipment that uses T1/DS1 signals for both data transport and network traceable timing recovery have used wired T1/DS1 circuits. These wired circuits were either carried directly as T1 circuits, or were derived from wired T3/DS3 electrical or SDH/SONET optical circuits, all of which are TDM circuits. For example, T1 or DS1 circuits can be generated from T3 or DS3 circuits, capable of carrying 28 T1s. DS3 circuits can be transported in optical networks as STS-1 circuits, in which 3 STS-1 circuits can be carried in an OC-3. The same STS-1 circuits can be carried in higher TDM optical circuits such as OC-12, OC-48, OC-192 or OC-768. Alternately, other technologies, such as asynchronous transfer mod (ATM) can be used to carry the T1 circuits. At the destination, reverse mappings can be employed to obtain timing recovery signals.
Hardwired TDM circuits—i.e., non-cellular systems—have been well characterized and specified in ANSI T1.403-1999—the American National Standard Institute for Telecommunications—Network and Customer Installation Interfaces—DS1 Electrical Interface. The Standards Committee T1 Telecommunications responsible for this document specified in section 6.3 the key aspects of jitter, wander, and phase transients that affect the ability of a receiver to recover data bits and track the recovered clock of the T1 signal. The key specifications are: maximum jitter of 5 UIpp (or unit intervals peak-to-peak) and 0.1 UIpp depending on jitter bandwidth; maximum network signal wander of 28 UI in a 24 hour period and 13 UI in a 15 minute interval; and maximum phase transients of 1.5 UI or instantaneous frequency shifts of 61 ppm. A unit interval is, in isochronous communication transmissions, the longest interval of which the theoretical durations of the significant intervals of a signal are all whole multiples. These key specifications enable telecommunication manufacturers to develop equipment that can be assured to interoperate.
The specifications for wander, both long term (24 hours) and short term (15 minutes), are defined as the wander measured against a primary reference source (PRS) detailed in ANSI T1.101. These specifications enable telecom engineers to develop synchronization algorithms which filter the jitter, phase transients, and wander of a T1 network interface to derive a clock that is suitable for global system for mobile communications (GSM) timing requirements.
As discussed, the important specifications for T1 wander from T1.403 (§6.3.1.2) for a T1 traffic interface are: <13 UIpp in 15 minutes; and <28 UIpp over 24 hours. Similar standards are specified for E1 (2.048 Mbps) circuits used in other parts of the world including Europe. The ETSI specification G.823 details these values in Table 2 (E1 traffic interface) as 18 microseconds (μs) over 1000 s.
FIG. 1 illustrates a combined hard wired communication network and wireless communication network for which are prescribed specifications of T1 wander for a primary reference source. In FIG. 1, primary reference source (PRS) 2, which is part of public switch telephone network (PSTN) 4a, is transmitted over T1 line 6a through mobile switching center (MSC) 8, base station controller (BSC) 10, second T1 line 6b, through interworking function (IWF) gateway 12a, through PSTN 4b and then IWF gateway 12b, wherein the maximum end-to-end wander, peak-to-peak of PRS 2, must be less than 28 unit intervals (UI).
Additional standards are published for timing T1 and E1 circuits. T1.101, section 7.2.1 defines a timing T1 reference input with wander specified to 1 μs in 30 minutes (about 2000 seconds), and 2 μs in 72 hours (about 100,000 seconds). ETSI specification G.823, Table 12, defines a plesiochronous digital hierarchy (PDH) synchronization interface with similar wander requirements of 2 μs in 2000 seconds and 5.33 μs in 100,000 seconds.
The standards described above for T1 and E1 timing circuits are defined largely for T1's generated by building integrated timing supply (BITS) units that are found in virtually all central offices. BITS are used to generate very high quality clocks to be fed to all telecommunications equipment requiring timing within a central office. These timing T1 circuits are expensive and are not available outside of central offices, and as such are not used for GSM base station timing applications.
GSM networks have specified rigorous timing requirements for the base transceiver stations (BTS) that are the wireless point of connectivity for mobile stations or more typically, cell phones. The clock used in the GSM base station must be traceable to the base station controller/mobile switching centre (BSC/MSC) to within an absolute accuracy of better than ±50 parts per billion (PPB). The BTS uses the recovered T1 to generate a recovered clock which meets the ±50 ppb specification and from that clock, derives the timing of both the cellular radio frequency carriers as well as the bit level timing of the GSM transmitted bits streams.
The effects of a clock error can be significant. An error of ±50 ppb will result in a frequency offset of approximately ±100 Hz, which on a 200 kHz RF carrier is not significant. However, an error of ±1000 ppb (or ±1 part per million—ppm) will yield a 2 kHz or 1% error in the RF channels and may result in adjacent channel interference and possibly non-compliances in the radio spectrum mask.
Whereas timing errors on the RF carriers can cause increased interference and radio non-compliances, timing errors in the GSM bit streams can result in network failures. GSM utilizes the recovered ±50 ppb clock to generate time division multiple access (TDMA) frames allowing up to eight mobile stations (MS) to maintain cellular calls with full rate coding, or sixteen MS using half rate coding. From GSM 05.01 v5.4.0, section 5, the ±50 ppb T1 derived clock is used to generate 3.69 μs bit periods, 0.577 millisecond (ms) timeslots, 4.515 ms TDMA frames, 120 ms multiframes (26 TDMA frames), 6.12 second superframes (51 multiframes), and 3 hour, 28 minute, and 53 second hyperframes (2048 superframes). Mobility handoffs rely on the synchronization of this chain of timings, all of which are derived from the recovered T1 clock. Included in these timing errors is the Doppler effect at the MS that accounts for high speed vehicular motion, and which translates into an effective timing error at the MS. At 1.9 GHz, the RF wavelength is 15 centimeters (cm), and a vehicle traveling at 120 km/hour will see an apparent clock error due to the Doppler effect of ±117 ppb. This error, added to the GSM requirement for ±50 ppb, yields ±167 ppb. The standard is defined to operate within up to ±300 ppb of effective timing error including Doppler effects before handovers start to fail. The GSM standard has been designed, assuming that ±50 ppb is achieved at the T1 timing interface, to support vehicular handoff at speeds up to 250 km/hour. The GSM standard was intended to address all land vehicular handoff cases; however, it did not account for the 350 km/hour super-trains found throughout Europe, and which demanded a revision of GSM—called GSM-R—to specifically to address these unique requirements.
GSM is an impressive mobility cellular network solution, but its operation is contingent upon the T1 clock derived from the PSTN. As long at this T1 interface meets the well defined wander and timing specifications for traffic T1 circuits, the network is designed to operate well. However, if the T1 circuits deviate from the timing specification, then deleterious results can be seen. If, for example, the T1 circuits deliver a ±200 ppb timing error for a short period of about 15 minutes, then an increase in MS handoff failures will occur for vehicles traveling above 100 km/hour (about 60 miles per hour). If the timing error exceeds ±250 ppb for the same period, then vehicles traveling faster than 50 km/hour will experience handoff failures. Finally, if the timing error exceeds ±300 ppb, then all vehicular traffic will experience mobility handoff failures, leaving the network to be operable only for pedestrian traffic.
Over the years, network operations centers (NOCs) have developed tools to monitor handoff failures and dropped calls, as an overall metric of network or BTS correct operation. NOCs typically employ a 3% dropped call rate to alarm mobility handoff and call setup problems, which can be caused by multiple factors. The dropped call alarm alerts the NOC staff to begin a time critical investigation to diagnose the root cause that may be traffic related, or network related. For example, a temporary increase in dropped calls may be indicative of an excessive traffic load beyond the designed parameters of the BTS site. Antenna failures or misalignments can result in increased dropped calls. Problems in adjacent BTS sites—power outages, or mean time between failure (MTBF) errors—can inadvertently affect traffic by shifting call traffic from one site to another. PSTN issues, such as loss of a T1 through a configuration error, can reduce traffic capacity and cause increased dropped calls. Finally, loss of stratum traceability, where any of the multiple switches and transport devices used to carry the T1 from the central office (CO) to the BTS site has entered holdover state, may result in handover failures. Cellular operators have developed alarms for all of these conditions, allowing a network level excessive dropped call rate to be logically and quickly diagnosed, keeping mean time to repair (MTTR) low. The introduction of pseudowire solutions for cellular backhaul has brought on a new set of network issues to diagnose. Unlike traditional TDM networks, which deliver T1 bit streams directly from the MSC/BSC to the CO, with well defined timing and data integrity requirements, pseudowire solutions rely on Ethernet/IP packet transport and clock regeneration at the BTS site. Whereas T1 TDM networks are specified with error rates better than 10−9 to yield a minimum of 72 hours of error free operation, packet networks were built on the premise of achieving network resiliency through network path switching, with little regard for delay variations caused by network topology changes, or for the loss of packets that occur during these transitions. As mentioned, T1 TDM networks are generally verified for 3 days of zero bit error rate for T1 circuits. The actual specification is given in specification ITU-T Recommendation G.826 “Error performance parameters and objectives for international, constant bit rate digital paths at or above the primary rate” which presents a BBER (block BER) 2*10-4 for a 27,000 km path or 1% of per 500 km, for a block size of 4632 bits (24 frames) which is equivalent to a BER of 4.31×10-8. Therefore, a target of 10-9 is considered acceptable performance for a T1 TDM circuit.
Other patents and published applications, as well as co-pending applications, include subject matter that can be considered related to the embodiments of the present invention below. Such documents include U.S. Published Patent Application No. 20070189164, entitled “System and Method for Packet Timing of Circuit Emulation Services Over Networks”, which details innovations in modifying the timing of pseudowire packet flows to largely eliminate micro-beating to enable very fine timing recovery such as is required for GSM networks; co-pending U.S. Non-provisional Utility patent application Ser. No. 12/027,894, entitled “Method and system for Controlling Link Saturation of Synchronous Data across Packet Networks”, which describes innovations in avoiding and limiting saturation conditions which can inadvertently affect the pseudowire data stream; and U.S. patent application Ser. No. 11/938,396, entitled “Network Delay Shaping System and Method for Backhaul of Wireless Networks”, which details network innovations for assuring absolute maximum delay of pseudowire derived traffic for wireless system, most specifically CDMA. Further, the following patents contain information related to T1 timing for GSM networks derived from a T1 signal: U.S. Pat. No. 6,104,915, entitled “Synchronization System Using Aging Prediction”; U.S. Pat. No. 6,178,215 entitled “Synchronization System for Reducing Slipping”; and U.S. Pat. No. 6,304,582 “Synchronization System Using Multiple Modes of Operation”.
It would be preferable for pseudowire solutions to adapt to the stochastic variations in network operation affecting the packet stream, including: packet jitter, phase transients, and wander. Such solutions cannot rely on external or absolute references such as a global positioning system (GPS) timing input since the derived pseudowire timing is required to follow the MSC/BSC. As a result, it would be highly advantageous for the pseudowire solution to dynamically adapt to these stochastic variations, and when not possible, raise alarms for conditions where the ±50 ppb T1 timing cannot be achieved. Until now, however, there have been no reliable means of defining and alarming conditions where the regenerated T1 circuit is unable to achieve the GSM requirement of ±50 ppb.